In recent years, III nitride such as gallium nitride (GaN) has been paid attention as an element that can realize a high-quality short-wavelength light emitting diode and a laser diode. There are many problems to be solved in putting electronic devices and others utilizing such a III nitride structure into practical use.
The growth technique of semiconductor crystal, for example, the epitaxial technique, the MOCVD (Metal Organic Chemical Vapor Deposition) technique, and others provide a controllability in the lamination direction; however, in order to make a structure in the in-plane direction, processing must be carried out using a different technique. The crystal processing techniques may be roughly classified into the top-down type in which the crystal is processed after the crystal growth and the bottom-up type in which the substrate is processed before the crystal growth and the structure is fabricated simultaneously with the crystal growth. By the top-down type, damages are given to the crystal by the processing, so that a problem arises particularly in a fine structure because the surface area will be large. On the other hand, the fabrication method for the bottom-up type can in many cases ensure both the controllability of the structure and the crystal quality.
In relation to the nitride semiconductor, there is a method for using a mask of silicon oxide or the like as a technique for fabricating a fine structure of the bottom-up type. This method in which the crystal is grown selectively in the opening of the mask that is patterned on the substrate is practically used in the vapor deposition method. However, by the molecular beam epitaxy method (which will be hereafter abbreviated as MBE), the polycrystal will be deposited on the mask.
M. Yoshizawa and others have found out a method for forming a gallium nitride crystal having a fine columnar shape with a diameter of about 100 nm in a self-organizing manner by growing gallium nitride under a condition of excessive nitrogen in the MBE using activated nitrogen excited by high-frequency plasma as a nitrogen source (See the non-patent document 1).
Further, H. Sekiguchi and others report that the diameter, the density, and the independence of gallium nitride fine columnar crystal that grows on a sapphire substrate with thin film aluminum nitride serving as a buffer layer are largely dependent on the surface morphology of the buffer layer (See non-patent document 1). The thin film aluminum nitride buffer layer grows into a shape having unevenness. The morphology thereof is dependent on the film thickness, and there is a tendency such that, when the film thickness is small, numerous small grains are formed, whereas when the film thickness is large, the grain size will be large. Further, the columnar gallium nitride crystal that has grown thereon has a tendency such that, according as the film thickness of aluminum nitride becomes large, the columnar gallium nitride crystals will have a small diameter and will be independent from each other. In the above report, a sapphire substrate is used. However, with regard to the other substrates as well, the morphology of the buffer layer and the shape of the columnar crystal that has grown thereon are greatly related with each other.
H. Tang and others have grown GaN selectively on a thin film AlN by fabricating a resist pattern using a photo-exposure technique after thin film AlN of 25 nm is fabricated on a Si (111) substrate by using the MBE method using ammonia as the crystal growth method, fabricating a pattern of AlN by selectively etching the thin film AlN, and further growing the crystal of GaN using the ammonia MBE method, and have verified that GaN does not grow in the part where AlN has been removed (See non-patent document 3). It is shown that, in the ammonia MBE, the growth nucleus forming temperature of GaN is higher on the AlN than on the Si, so that selective growth of GaN can be realized when the growth is carried out at a suitable temperature.
T. Martensson and others use particulate Au patterned on an InP substrate as a catalyst and fabricate an InP nano-wire with a controlled shape by regular arrangement through VLS mode growth (See patent document 4).    Non-Patent Document 1: M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, and K. Kishino, Jpn. J. Appl. Phys. Vol. 36 (1997), pp. L459-L462    Non-Patent Document 2: H. Sekiguchi, T. Nakazato, A. Kikuchi, and K. Kishino, Journal of Crystal Growth    Non-Patent Document 3: H. Tang, S. Haffouz, and J. A. Bardwell, Applied Physics Letters 88. 172110 (2006)    Non-Patent Document 4: T. Martensson, P. Carlberg, M. Borgstrom, L. Montelius, W. Seifert, and L. Samuelson, Nano Letters 4, 699 (2004)